On Wed, Feb 10, 2016 at 2:32 AM, Jan Kara <jack@xxxxxxx> wrote: > On Tue 09-02-16 10:18:53, Dan Williams wrote: >> On Tue, Feb 9, 2016 at 9:24 AM, Jan Kara <jack@xxxxxxx> wrote: >> > Hello, >> > >> > I was thinking about current issues with DAX fault locking [1] (data >> > corruption due to racing faults allocating blocks) and also races which >> > currently don't allow us to clear dirty tags in the radix tree due to races >> > between faults and cache flushing [2]. Both of these exist because we don't >> > have an equivalent of page lock available for DAX. While we have a >> > reasonable solution available for problem [1], so far I'm not aware of a >> > decent solution for [2]. After briefly discussing the issue with Mel he had >> > a bright idea that we could used hashed locks to deal with [2] (and I think >> > we can solve [1] with them as well). So my proposal looks as follows: >> > >> > DAX will have an array of mutexes (the array can be made per device but >> > initially a global one should be OK). We will use mutexes in the array as a >> > replacement for page lock - we will use hashfn(mapping, index) to get >> > particular mutex protecting our offset in the mapping. On fault / page >> > mkwrite, we'll grab the mutex similarly to page lock and release it once we >> > are done updating page tables. This deals with races in [1]. When flushing >> > caches we grab the mutex before clearing writeable bit in page tables >> > and clearing dirty bit in the radix tree and drop it after we have flushed >> > caches for the pfn. This deals with races in [2]. >> > >> > Thoughts? >> > >> >> I like the fact that this makes the locking explicit and >> straightforward rather than something more tricky. Can we make the >> hashfn pfn based? I'm thinking we could later reuse this as part of >> the solution for eliminating the need to allocate struct page, and we >> don't have the 'mapping' available in all paths... > > So Mel originally suggested to use pfn for hashing as well. My concern with > using pfn is that e.g. if you want to fill a hole, you don't have a pfn to > lock. What you really need to protect is a logical offset in the file to > serialize allocation of underlying blocks, its mapping into page tables, > and flushing the blocks out of caches. So using inode/mapping and offset > for the hashing is easier (it isn't obvious to me we can fix hole filling > races with pfn-based locking). > > I'm not sure for which other purposes you'd like to use this lock and > whether propagating file+offset to those call sites would make sense or > not. struct page has the advantage that block mapping information is only > attached to it, so when filling a hole, we can just allocate some page, > attach it to the radix tree, use page lock for synchronization, and allocate > blocks only after that. With pfns we cannot do this... Right, I am thinking of the direct-I/O path's use of the page lock and the occasions where it relies on page->mapping lookups. Given we already have support for dynamically allocating struct page I don't think we need to have a "pfn to lock" lookup in the initial implementation of this locking scheme. -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
